1. Field of the Invention
The present invention relates to gas turbine engines and more particularly to airfoils used in engines having high turbine inlet temperatures.
2. Description of the Prior Art
The design and construction of gas turbine engines has always required precise engineering effort to ensure the structural integrity of the individual components. One particularly critical area for concern is the turbine nozzle which is formed of a plurality of vanes disposed across the flow path for the working medium gases. During operation of the engine, the flowing medium is redirected by the nozzle onto the rotor blades of the turbine wheel. The temperature of the medium at the inlet to the turbine normally exceeds the allowable temperature limit of the material from which the vanes are fabricated. Conventionally, the vanes are cooled to prolong their service life by reducing the temperature of the vane material during operation.
In most constructions vane cooling air is directed from the compressor, through various conduit means both inwardly and outwardly of the working medium flow path and to the turbine section of the engine. A hollow cavity within the airfoil section of each vane receives the cooling air. Air entry ports at both ends of the hollow cavity are in communication with the conduit means. A typical vane utilized in a cooled turbine is shown in U.S. patent application, Ser. No. 531,632, entitled, "Cooled Turbine Vanes" by Leogrande et al, of common assignee herewith. In Leogrande a U-shaped insert is disposed within a hollow cavity at the leading edge of the airfoil section. The cooling air is accelerated and directed by small diameter holes in the insert to velocities at which the flow impinges upon the cavity walls. The air is subsequently flowed over the outer walls of the airfoil section to film cool the outer surfaces of the vane.
Film cooling requires a precise, relatively low pressure differential across the flow emitting holes. If the pressure differential is too high, the emitted flow penetrates the passing medium and is directed downstream with the combustion gases without adhering to the airfoil surface. On the other hand, if the pressure differential is too small, the hot medium gases penetrate the cooling air layer adjacent the vane and cause destructive heating of the vane material. Because the required pressure differential between the working medium gases and the cooling air within the vane cavity is relatively small, the amount of flow over the walls is highly sensitive to local pressure deviations within the cavity.
To implement the conjunctive use of impingement and film cooling, continuing efforts are being directed to provide apparatus which will isolate cooling air to the pressure side chambers of the vane from cooling air to the suction side chambers of the vane.